There are two basic types of bearing assemblies used for down-hole drilling motors in the oil and gas industry: sealed assemblies and mud-lubricated assemblies.
The oil sealed assembly uses seals to separate an oil chamber from the drilling fluid. This style is fitted with common precision bearings within the oil chamber to accommodate the thrust and radial loads required for the drilling processes. These bearings are designed to operate in a fluid that is free of particles that cause wear. It is essential that the seals separating the drilling fluid from the oil do not fail. Invasion of the oil chamber by drilling fluid causes severe damage to the precision bearing components and leads to premature failure of the drilling motor. This occurrence is expensive and can require the replacement of the drilling motor to continue drilling operations.
The mud lubricated bearing assembly does not use an oil sealed chamber. The load bearing components are exposed to a small flow of the drilling fluid to keep them cool and lubricated. They are usually less precise in their manufacture, and are expected to wear and be replaced regularly. One major disadvantage of the mud lubricated system is the load carrying capacity of the thrust bearings. To provide an equivalent thrust capacity in a mud lubricated assembly, several rows of roller balls and races, termed a “stack”, must be utilized to equal the equivalent capacity of a single precision oil sealed bearing. In addition, the alternating rows of balls and races must share the load equally to be effective. As manufacturing these stacks with sufficient accuracy to ensure equal load sharing was found to be expensive and difficult, the stacks were manufactured to a reasonable economic accuracy and allowed to “wear-in” to achieve the load sharing. The bearing were able to wear-in due to the particles (abrasive solids), present in drilling fluids, which allowed load sharing to be quickly achieved in use and once achieved, provided a reasonably useful life for the mud lubricated stack. This type of wear-in cannot be tolerated in an oil sealed bearing assembly due to the much increased load requirements of a single bearing.